1. Field of the Invention
The present invention relates to an asymmetrical vertical bidirectional component of protection against overvoltages. It more specifically relates to a protection component formed of three semiconductor layers of alternated conductivity types (only PNP structures will be mentioned hereafter to simplify the present description, but the invention also applies to NPN structures).
2. Discussion of the Related Art
Bidirectional protection components of the type having three semiconductor layers of alternated conductivity type are well known. In this type of component, the gain of the transistor formed by the PNP layers may be too low for this transistor to be able to trigger, that is, this component may behave only as two head to tail diodes in series.
Various structures have been provided to form a bidirectional protection diode with three semiconductor layers of alternated biasings.
FIG. 1 shows a first embodiment of a bidirectional protection component. This component is formed from an N-type semiconductor substrate 1.
On each side of the substrate are formed, generally by implantation/diffusion, opposite heavily-doped P-type areas 2 and 3, respectively in contact with metallizations 4 and 5. The upper and lower (or front surface and rear surface) peripheries of the component are coated with an insulating layer, typically silicon oxide, respectively 6 and 7.
The bidirectional protection component shown in FIG. 1 is very simplified. In practice, it will comprise various areas intended to improve its voltage and peripheral behavior, for example, heavily-doped N-type channel stop regions, on the upper side and on the lower side. Conventionally, during its manufacturing, this component forms an element of a semiconductor wafer which is then sawn as shown in FIG. 1.
The bidirectional protection component shown in FIG. 1 has a particularly high performance. According to the desired protection voltage, N-type substrate 1 will be more or less heavily doped and symmetrical protection voltages ranging from 6.8 to 220 volts can thus be obtained. Further, given that the component is formed from a relatively thick silicon substrate 1, for example, with a thickness ranging from 200 to 300 μm, the parasitic PNP transistor will have a particularly low gain and does not risk turning on, at the cost, however, of a non-negligible series resistance.
However, this component has a disadvantage in terms of assembly. Indeed, the lower surface of the component may not be able to be welded to a planar conductive base since any wicking could then risk short-circuiting metallization 5 and substrate 1. A base comprising a pedestal substantially having the surface of metallization 5 should be provided to avoid any weld overflow towards substrate 1. Such a configuration may be incompatible with the assembly of modern micropackages.
It has thus been tried to form bidirectional protection components capable of being assembled on planar conductive bases.
FIG. 2 shows an example of a structure adapted to such an assembly, currently called a well structure. The component of FIG. 2 is formed from a heavily-doped P-type silicon wafer 11 (P+) on which an N-type layer 12, having a thickness ranging from 10 to 30 μm, is formed by epitaxy. A central area of layer 12 of a chip is covered with a heavily-doped P-type layer 13. The periphery of the epitaxial layer of a chip is surrounded with a P-type peripheral wall 14. P area 13 is coated with a metallization 16 and the lower surface of the substrate is coated with a metallization 17. An insulating layer 18, for example, made of silicon oxide, covers the upper periphery of the chip.
The structure of FIG. 2 effectively solves the problem of the welding of the chip on a planar conductive wafer. Indeed, even if there is a welding overflow, given that the entire periphery of the chip is of type P, no short-circuit risks to occur.
The structures of FIGS. 1 and 2 have substantially symmetrical breakdown voltages in the two biasings with which they are used, since the two useful junctions are formed from the same lightly-doped N layer. A light asymmetry (at most on the order of 8 V) can be caused by varying the doping profiles of the junction between the layer or the N-type substrate and the P-type regions.
Let us mention as a reference unpublished French patent application of the applicant filed under number 10/53680 of May 11, 2010, which aims at forming a perfectly symmetrical bidirectional protection component.
Thus, known bidirectional protection components typically are as symmetrical as possible. To obtain asymmetrical bidirectional protections, the series connection of two discrete diodes of opposite biasing would generally be used.